1. Field of the Invention
The present invention relates to a fabricating method of a nitride semiconductor substrate. More particularly, the present invention relates to a method of fabricating nitride semiconductor substrate of low defect density and a composite material substrate with patterned structure which is fabricated with the method.
2. Description of Related Art
In recent years, GaN and related ternary compound semiconductors are widely applied to short wavelength optoelectronic devices and high-power high-frequency devices, however, due to the difficulties in fabricating GaN substrate, such semiconductors are generally grown on sapphire substrate and SiC substrate. Even though GaN monocrystal can be successfully grown on these two substrates through heteroepitaxy technology, high density defects are usually produced during epitaxy process due to large lattice mismatch, and such defects will limit the application and development of GaN material in optoelectronic semiconductor devices.
In General, due to the limitation in solubility and diffusibility of nitrogen in liquid gallium, it is very difficult to fabricate GaN substrate by using conventional single crystal growth technology. Thus, in recent years, hydride vapor phase epitaxy (HVPE) is developed and used for increasing the thickness of GaN on sapphire substrate greatly, so as to grow GaN thick film, however, the defect density and macro-cracking can not be reduced effectively, and the main factor thereof is still caused by the differences between lattice constants and coefficients of thermal expansion (CTE) existing in hetero-materials.
Presently, some patents for fabricating low defect density GaN substrate have been issued already, such as U.S. Pat. No. 6,964,914. In this patent, first, H+ implantation is performed to GaN or AlN monocrystal base material, and the implantation depth is the thickness of GaN after subsequent transferring. Then, the thin GaN layer is transferred onto other supporting substrate through direct-wafer-bonding or intermediate-wafer-bonding after the implantation process, and the transferred monocrystal layer is referred to as nucleation layer. Next, a thick GaN monocrystal layer is grown through HVPE. Finally, the GaN thick film and the supporting substrate are separated.
However, the foregoing U.S. patent has some disadvantages even though it can be used for fabricating free standing GaN thick film, for example, the bonding temperature up to 800˜1000° C., and the separating temperature of nucleation layer is also up to 900˜950° C., high temperatures may cause GaN or the supporting substrate to burst due to the difference in CTE. In addition to this, the present cost of GaN substrate is up to US$10,000.